Fibre membrane filtration is a well developed method which involves the use of a large number of hollow tubular micro-porous fibres, each fibre being adapted to allow filtrate to flow from the exterior of the fibre via micro-pores in the fibre wall to the interior of the fibre while excluding impurities from the filtrate. The pores can be, for example, around 0.2 micrometres in diameter.
In practice, many thousands of fibres are bundled together and encased in a shell with the assembly being known as a module. The shell is usually cylindrical and the fibres extend longitudinally of the shell with the ends of the shell being sealed, usually with a resin or the like known as the potting forming a plug at each end. The ends of the hollow fibres extend through, and are encased in the potting plug so that the interior of the fibres is in communication with the exterior of the module at both ends thereby allowing filtrate to be removed from two end locations. Alternatively, both ends of each fibre may extend through the potting and communicate with the exterior at one end of the module, or the fibres at one end may extend through the potting, the other fibre ends being sealed.
In practice modules are usually (but not necessarily) disposed in "banks", each comprising a row of modules sharing a manifold, the banks being arranged in an array.
In use, feed or influent is introduced to the space intermediate the exterior of the fibres and the interior of a module shell. Filtrate flows through the micro-porous membrane of the fibres into the interior of the fibres and thereafter flows along the length of the fibres passing through the plug to the exterior of the module, usually into a manifold.
A wastewater filtering system may comprise several hundred modules, each module containing many thousands of fibres. Although failure in these systems is rare, the failure or breakage of a single fibre may compromise the integrity of the entire system by allowing unfiltered influent to enter the interior of the failed fibre and thereby contaminate the filtrate.
A fibre may fail for example due to the presence of a sharp or hard particle in the influent, or for other reasons. On such occasions a difficulty arises in identifying the failed fibre within the many thousands of fibres in the system.
A known test for identifying a module containing a failed fibre is the Diffusive Air Flow Test (DAF). In this test the space intermediate the fibres and shell of a selected module or modules is filled with air and the interior of the fibres is supplied with pressurized air. The fibre membrane is wetted to fill the pores with liquid and the rate at which air diffuses from the interior of the fibre to the exterior of the fibre is measured. In the absence of any failed fibres the rate will correlate with a reference value which is indicative of the expected diffusive flow from the fibre interior to the fibre exterior at a given pressure difference for the particular membrane.
It is important to note that if a certain pressure is exceeded diffusive flow will be upset by the creation of bubbles of air at the pores. This is known as the bubble point of the membrane and is defined by the following equation: ##EQU1##
where P=Bubble Point Pressure
.theta.=wetting angle PA1 .beta.=Bechold capillary constant PA1 .gamma.=surface tension of wetting liquid PA1 d=diameter of pore PA1 supplying gas under pressure to the space intermediate the fibres and the shell and wetting the ends of the fibres; PA1 monitoring the ends of the fibres for the formation of bubbles indicative of a failed fibre; and PA1 sealing the failed fibre. PA1 filling the space intermediate the fibres and the shell or shells of the subset with liquid; PA1 supplying gas at a pressure below the nominal bubble point of the membrane to the interior of the fibres of the subset; PA1 measuring the rate of displacement of liquid from the space intermediate the fibres and the shell or shells of the subset caused by transfer of the gas from the interior to the exterior of the fibres; PA1 comparing the measured rate of displacement of liquid to a reference rate of displacement; and PA1 utilizing the comparison to assess whether a failed fibre is present in the subset. PA1 filling the space intermediate the fibres and the shell or shells of the set with liquid; PA1 supplying gas at a pressure below the nominal bubble point of the membrane to the interior of the fibres of the set; and PA1 using transducer means to assess whether bubbles are being formed in a subset of the set.
Clearly if a failed fibre is present the measured rate will be higher as it will include a component due to flow of air through the fault as opposed to diffusive flow through the pores.
Whilst this method is useful in identifying a module containing a failed fibre it has some disadvantages and does not extend to identifying the individual failed fibre.
It is an object of the invention to at least ameliorate some of the disadvantages of the prior art.